execute a function when is execute an app

[Silvio.Falconi]

I wish execute a function when is execute an app

I explain you

The coach driver has a program to make the tickets to the passengers.

If a passenger already has a subscription ie a ticket where a subscription is described and has a bar code.

I want the driver having the application open and ready to make the tickets have the ability to read the bar code of the passenger's subscription.

How can I do ?